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Accurate dose measurement tools are needed to evaluate the radiation dose delivered to patients by using modern and sophisticated radiation therapy techniques. However, the adequate tools which enable us to directly measure the dose distributions in three-dimensional (3D) space are not commonly available. One such 3D dose measurement device is the polymer-based dosimeter, which changes the material property in response to radiation. These are available in the gel form as polymer gel dosimeter (PGD) and ferrous gel dosimeter (FGD) and in the solid form as solid plastic dosimeter (SPD). Those are made of a continuous uniform medium which polymerizes upon irradiation. Hence, the intrinsic spatial resolution of those dosimeters is very high, and it is only limited by the method by which one converts the dose information recorded by the medium to the absorbed dose. The current standard methods of the dose quantification are magnetic resonance imaging, optical computed tomography, and X-ray computed tomography. In particular, magnetic resonance imaging is well established as a method for obtaining clinically relevant dosimetric data by PGD and FGD. Despite the likely possibility of doing 3D dosimetry by PGD, FGD or SPD, the tools are still lacking wider usages for clinical applications. In this review article, we summarize the current status of PGD, FGD, and SPD and discuss the issue faced by these for wider acceptance in radiation oncology clinic and propose some directions for future development.

One of treatment planning techniques with Leksell GammaPlan (LGP) for Gamma Knife stereotactic radiosurgery (GKSRS) uses multiple matrices with multiple dose prescriptions. Computational complexity increases when shots are placed in multiple matrices with different grid sizes. Hence, the experimental validation of LGP calculated dose distributions is needed for those cases. For the current study, we used BANG3 polymer gel contained in a head‐sized glass bottle to simulate the entire treatment process of GKSRS. A treatment plan with three 18 mm shots and one 8 mm shot in separate matrices was created with LGP. The prescribed maximum dose was 8 Gy to three shots and 16 Gy to one of the 18 mm shots. The 3D dose distribution recorded in the gel dosimeter was read using a Siemens 3T MRI scanner. The scanning parameters of a CPMG pulse sequence with 32 equidistant echoes were as follows: TR=7s, echo step = 13.6 ms, field‐of‐view = 256 mm× 256 mm, and pixel size=1 mm×1 mm. Interleaved acquisition mode was used to obtain 15 to 45 2‐mm‐thick slices. Using a calibration relationship between absorbed dose and the spin‐spin relaxation rate (R2), we converted R2 images to dose images. MATLAB‐based in‐house programs were used for R2 estimation and dose comparison. Gamma‐index analysis for the 3D data showed gamma values less than unity for 86% of the voxels. Through this study we accomplished the first application of polymer gel dosimetry for a true comparison between measured 3D dose distributions and LGP calculations for plans using multiple matrices for multiple targets. PACS number: 87.53.Ly, 87.55‐x, 87.56 ‐g

Verification of the dose calculation model and the software used for treatment planning is an important step for accurate radiation delivery in radiation therapy. Using BANG3 polymer gel dosimeter with a 3 Tesla magnetic resonance imaging (MRI) scanner, we examined the accuracy of TomoTherapy treatment planning and radiation delivery. We evaluated one prostate treatment case and found the calculated three‐dimensional (3D) dose distributions agree with the measured 3D dose distributions with an exception in the regions where the dose was much smaller (25% or less) than the maximum dose (2.5 Gy). The analysis using the gamma‐index (3% dose difference and 3 mm distance‐to‐agreement) for a volume of 12 cm×11 cm×9 cm containing the planning target volume showed that the gamma values were smaller than unity for 53% of the voxels. Our measurement protocol and analysis tools can be easily applied to the evaluation of other newer complex radiation delivery techniques, such as intensity‐modulated arc therapy, with a reasonably low financial investment. PACS numbers: 87.53Bn, 87.56Fc

Gel dosimetry is a promising technique for the implementation of 3D dose verification within the radiation therapy clinic, since it is the only methodology which provides comprehensive 3D dose measurement of conformal treatments such as Intensity Modulated Radiation Therapy (IMRT), Stereotactic radiosurgery (SRS) and Stereotactic Radiotherapy (SRT), and volumetric arc therapies (VMAT). In polymer gel dosimetry, monomer molecules polymerize and are fixed in the gel matrix when exposed to radiation. The spatial dose information thus can be obtained by gel imaging. Gel imaging methods which are currently under investigation are MRI, optical computed tomography and x-ray computed tomography. While promising, x-ray CT images of irradiated polymer gel exhibit low contrast due to small change in density that occurs during polymerization. The response of CT contrast to dose is weak and noise reduction is critical in order to achieve adequate dose resolution in gel dosimetry using x-ray computed tomography. Ideally, image noise is minimized using high tube current, long scan times and high number of image averages [Hilts et al 2005]. Digital image filtering is an effective method in reducing image noise while maintaining accurate spatial dose information. Generally it is performed in either frequency domain or spatial domain. In a previous work, several spatial filters were applied to Stereotactic Radiosurgery (SRS) irradiated polymer gel image [Hilts and Duzenli 2004]. In their study it was found that the two highest performing filters were the adaptive mean (Wiener) and smallest univalue segment assimilating nucleus (SUSAN). In another study a new method of signal to noise ratio (SNR) enhancement by 2D two-point maximum entropy regularization method (TPMEM) was investigated [Jirasek 2006]. However, a comprehensive examination of the best performing filters from these two studies has not been undertaken. Here, we test the Weiner and TPMEM digital filters on X-ray CT image of the irradiated PAGAT gel in order to reduce noise and improve dose resolution. This work builds off of previous studies and evaluates and compares the highest-performing filters from past individual works [Hilts and Duzenli 2004, Jirasek et al 2006].

Purpose: Aim of the study is to evaluate mechanical and radiological accuracy of multi-fraction regimen and validate Gamma knife based fractionation using newly developed patient simulating multipurpose phantom. Methods: A patient simulating phantom was designed to verify fractionated treatments with extend system (ES) of Gamma Knife however it could be used to validate other radiotherapy procedures as well. The phantom has options to insert various density material plugs and mini CT/MR distortion phantoms to analyze the quality of stereotactic imaging. An additional thorax part designed to predict surface doses at various organ sites. The phantom was positioned using vacuum head cushion and patient control unit for imaging and treatment. The repositioning check tool (RCT) was used to predict phantom positioning under ES assembly. The phantom with special inserts for film in axial, coronal and sagittal plane were scanned with X-Ray CT and the acquired images were transferred to treatment planning system (LGP 10.1). The focal precession test was performed with 4mm collimator and an experimental plan of four 16mm collimator shots was prepared for treatment verification of multi-fraction regimen. The prescription dose of 5Gy per fraction was delivered in four fractions. Each fraction was analyzed using EBT3 films scanned with EPSON 10000XL Scanner. Results: The measurement of 38 RCT points showed an overall positional accuracy of 0.28mm. The mean deviation of 0.28% and 0.31 % were calculated as CT and MR image distortion respectively. The radiological focus accuracy test showed its deviation from mechanical center point of 0.22mm. The profile measurement showed close agreement between TPS planned and film measured dose. At tolerance criteria of 1%/1mm gamma index analysis showed a pass rate of > 95%. Conclusion: Our results show that the newly developed multipurpose patient simulating phantom is highly suitable for the verification of fractionated stereotactic radiosurgery using ES of Gamma knife. The study is a part of intramural research project of Research Section, All India Institute of Medical Sciences New Delhi India (A 247).

Purpose: To evaluate forward and inverse planning methods for acoustic neuroma cases treated in Gamma Knife Perfexion. Methods: Five patients with acoustic neuroma tumour abutting brainstem were planned twice in LGP TPS (Version 10.1) using TMR10 algorithm. First plan was entirely based on forward planning (FP) in which each shot was chosen manually. Second plan was generated using inverse planning (IP) for which planning parameters like coverage, selectivity, gradient index (GI) and beam-on time threshold were set. Number of shots in IP was automatically selected by objective function using iterative process. In both planning methods MRI MPRAGE sequence images were used for tumour localization and planning. A planning dose of 12Gy at 50% isodose level was chosen. Results and Discussion: Number of shots used in FP was greater than IP and beam-on time in FP was in average 1.4 times more than IP. One advantage of FP was that the brainstem volume subjected to 6Gy dose (25% isodose) was less in FP than IP. Our results showed use of more number of shots as in FP results in GI less than or equal to 2.55 which is close to its lower limit. Dose homogeneity index (DHI) analysis of FP and IP showed average values of 0.59 and 0.67 respectively. General trend in GK for planning in acoustic neuroma cases is to use small collimator shots to avoid dose to adjacent critical structures. More number of shots and prolonged treatment time causes inconvenience to the patients. Similarly overuse of automatic shot shaping as in IP results in increased scatter dose. A compromise is required in shot selection for these cases. Conclusion: IP method could be used in acoustic neuroma cases to decrease treatment time provided the source sector openings near brainstem are shielded or adjusted appropriately to reduce brainstem dose.

Purpose: Gamma Knife perfexion (GKPFX) with extend system is being used for multi-fraction or fractionated stereotactic radiosurgery (fGKSRS). Patient surveillance Unit (PSU) is used to attain a non invasive immobilization for fractionation. Calibrated digital probe is used to measure encoded mechanical positions on repositioning check tool (RCT). The objective of this study is to check consistency and statistical position variation during multi-fraction stereotactic radiosurgery using GKPFX extend system. Methods: 31 fractions through 7 patients being delivered to check the positional variation during patient setup. Variation is calculated after sampling the collected data at same point measurement for all the patients. Average value of three stage measurements or the reference values are compiled with the sample data using SPSS (statistical software). Measurement of encoded points has been taken and various statistical parameters are observed. Reproducibility is checked, comparing the reference to measured values through encoded points before treatment. Intra-fraction variation is measured on compiling pre and post treatment RCT measurements while inter-fractionation variation compiled along with fractions before treatment. Results: Pre-treatment radial setup difference auto calculated by the system varies from 0.5mm to 0.8 mm with mean variation of 0.7 mm. Positional variation was observed for post-treatment and was found that it varies from 0.9 mm - 1.0 mm. Paired sample test results the paired difference mean from the reference (for pre and post measurements) was 0.26 mm (0.1775 mm- 0.3475 mm) with only 25% of error significance. The distribution of the random component of observer uncertainty was 0.06mm. Conclusion: Post treatment positional variation is 20%-30% of pre treatment measurements, i.e. about treatment measurements, i.e. about 0.2 mm. Explaining complete procedure to patient, limiting head rotation/lift and preventing other movements, may maximize positional accuracy during fractionations. Contending positional accuracy, supports the extend system for fractionated stereotactic radiosurgery among other multisession radiosurgery techniques.

Purpose: To validate the three‐dimensional (3D) dose distributions calculated by Leksell Gamma Plan (LGP) for Gamma Knife stereotactic radiosurgery. (GKSRS) cases with multiple matrices using MRI‐based polymergeldosimetry.Method and Materials: BANG3 polymergel contained in a head size glass bottle simulated the entire treatment process of GKSRS. All phantoms were fixed in the Leksell stereotactic frame. For the first study one phantom was used for calibration and the second was irradiated with five 18‐mm shots in five dose matrices. For the second study the calibration was performed with 8‐mm shots and a treatment plan was generated with three 18‐mm shots and one 8‐mm shot. The 3D‐dose distribution recorded in the gel dosimeter was read using a 3T MRI scanner (Siemens Magnetom Trio). Scanning parameters were CPMG pulse sequence with 32 equidistant echoes, TR = 7 s, echo step = 13.6 ms, pixel size = 1mm × 1mm. Interleaved acquisition mode was used to obtain 15 to 45 2mm‐thick slices. Using a calibration relationship between absorbed dose and spin‐spin relaxation rate (R2), we converted R2 images to dose images. The dose comparison was accomplished using an in‐house MATLAB‐based program. Results:Calibrations indicate the dose response of the current polymergel was non‐linear for doses higher than 10 Gy. The artifacts mainly caused by the metallic pins used to immobilize the phantom distorted the measured dose distributions. Away from the pin positions, however, we could compare the LGP dose distributions with measured dose distributions. Preliminary analyses indicated some spatial position discrepancies in addition to the dose differences. Conclusion: Through this study we could perform a true comparison between 3D dose distributions measured with a polymergel dosimeter and LGP calculations for plans using multiple matrices/targets.

Purpose:Doses to rectum, bladder and Point‐A in patients who received intracavitary HDR brachytherapy treatment were calculated by both orthogonal radiographsreconstruction method and by image based reconstruction method and were compared. Method and Materials: Twenty‐five patients of carcinoma cervix who received intracavitary brachytherapy treatment by micro‐Selectron HDR unit were included in this study. For these patients, in Plato treatment planning system, reconstruction of sources, applicator, organs at risk and Point‐A were done using conventional orthogonal radiographs and by using 1 mm thick CT scanimages. Bladder was identified on radiographs through a Foley's catheter inserted into it filled with 7 cc of radio‐opaque fluid and the rectum through a rectal marker. Doses were calculated at 5 points each on bladder and rectum, for the treatment dose of 7 Gy to Point‐A (right & left). On CTimages, these organs were delineated with the radiologists help and from dose‐volume‐histograms,doses to 1, 2 and 5 cc of these organs were calculated besides dose to Point‐A (right & left). Results: The critical organdoses estimated by the CT method were consistently higher when compared to those by the radiographs method. When compared to the radiographicdose maximum for bladder, doses to 1, 2 and 5 cc of bladder were higher by 89%, 69.7% and 41.6% on an average respectively. For rectum, the corresponding values were 36.6%, 21.7% and 0.4%. However dose to Point‐A did not differ by more than 3% between the two methods. Conclusion: The conventional orthogonal radiography based reconstruction method consistently underestimates the doses to critical organs while the doses to reference points are comparable in these two methods. Long‐term follow‐up in such patients can identify the threshold volumes for these critical organs beyond which complication rates are bound to rise.

Purpose: To present EBT2 film verification of treatment planning with the eXtend System, a relocatable frame system for multiple‐fraction or serial multiple‐session radiosurgery.Methods: A human head shaped phantom simulated the verification process for fractionated Gamma Knife (GK) treatment. Phantom preparation for eXtend Frame based treatment planning involved creating a dental impression, fitting the phantom to the frame system, acquiring a stereotactic computed tomography(CT) scan. A CT scan (Siemens, Emotion6) of the phantom was obtained with following parameters: Tube Voltage ‐ 110 kV, Tube Current ‐ 280 mA, pixel size ‐ 0.5 mm × 0.5 mm and 1 mm slice thickness. A treatment plan with two 8 mm collimator shots and three sector blocking in each shot was made. Dose prescription of 4.0 Gy at 100% was delivered for the first fraction out of the two fractions planned. Gafchromic EBT2 film (ISP Wayne, NJ) was used as 2D verification dosimeter in this process. Films were cut and placed inside the film insert of the phantom for treatment dose delivery. Meanwhile a set of films from the same batch were exposed from 0 Gy to 12 Gy doses for calibration purpose. EPSON (Expression 10000XL) scanner was used for scanning the exposed films in transparency mode. Scanned films were analyzed with in‐house made Matlab codes. Results: Gamma index analysis of film measurement in comparison with TPS calculated dose resulted in high pass rates >90% for different tolerance criteria of 2%/2mm, 1%/1mm, and 0.5%/0.5mm. The isodose overlay and linear dose profiles of film measured and computed dose distribution on sagittal and coronal plane was in close agreement. Conclusions: Through this study we propose a treatment verification QA method for eXtend frame based fractionated Gamma Knife radiosurgery using EBT2 film. Acknowledgement: Authors acknowledge the help of Andre Micke, ISP for sharing his expertise on EBT2 film.